FIG. 13 is a partial cross-sectional view of a conventional electric blower. The electric blower includes: motor 2 having rotary shaft 1, impeller 4, air guide 5, and fan case 6. Impeller 4 is secured to rotary shaft 1 by nut 3 and rotationally driven by motor 2. Air guide 5 converts flow energy of air, exhausted from impeller 4, into pressure energy. Fan case 6 accommodates impeller 4 and air guide 5.
FIG. 14 is a partial cross-sectional view of the impeller of the conventional electric blower. Impeller 4 is configured with sheet-metal rear shroud 11, front shroud 12, a plurality of sheet-metal blades 13, and resin inducer 15. Front shroud 12 is disposed with a space from rear shroud 11, and is a sheet-metal one. Sheet-metal blades 13 are fitted to and fixed between a pair of rear shroud 11 and front shroud 12. Resin inducer 15 is disposed corresponding to suction opening 14 disposed at the center of front shroud 12. Sheet-metal blades 13 are secured by calking to rear shroud 11 and front shroud 12. Moreover, resin inducer 15 is configured with hub 16 of an approximate cone shape and blade parts 17 formed on hub 16. Especially, each of blade parts 17 is of a shape having a three-dimensional curved surface so as to rectify air that flows from suction opening 14 toward sheet-metal blades 13.
FIG. 15A is a plan view of the structure of a mold for an inducer of the conventional electric blower. FIG. 15B is a side elevational view of the structure of the mold for the inducer of the electric blower. In order to obtain such a complex form, inducer 15 is formed by resin-molding which employs side-sliding molds 21 that slide approximately radially in the direction from the center toward the outer periphery sides of blade parts 17. The mold is configured with core 22, cavity 23, and side-sliding molds 21 corresponding in number to blade parts 17 (see Patent Literature 1, for example).
FIG. 16 is a partial cross-sectional view of a conventional electric blower having another configuration. As shown in FIG. 16, inducer 31 has a vertical two-way-split configuration that includes first inducer 31a and second inducer 31b. First inducer 31a and second inducer 31b are tightened together and secured to rotary shaft 33 by nut 32 (see Patent Literature 2, for example).
Moreover, FIG. 17A is a cross-sectional view of an inducer of a conventional electric blower having further another configuration. FIG. 17B is a cross-sectional view taken along line 17B-17B in FIG. 17A. Inducer 41 has a vertical two-way-split configuration that includes first inducer 41a and second inducer 41b. Recesses 43 are disposed in blade parts 42a of first inducer 41a, while projections 44 are disposed on blade parts 42b of second inducer 41b. Projections 44 are fitted with recesses 43 by shrinkage-fit, thereby securing second inducer 41b to first inducer 41a (see Patent Literature 3, for example).
In Patent Literature 1, the number of the blade parts is optimally set to six in view of the relation between the number of the blade parts and fan efficiency. However, in consideration of air-flow volume and the number of rotations, there are sometimes cases where a multi-blade configuration having more than six blade parts is preferable. Moreover, high-frequency sounds, i.e. a kind of noise generated by the electric blower, are generated outstandingly at frequencies equal to integral multiples of the product of the number of the blade parts and the number of rotations. When the number of the blade parts is small, some of the frequencies are in an audibility range of human ears, with the frequencies being equal to the integral multiples of the product of the number of the blade parts and the number of rotations. This causes nagging noises grating on user's ears; therefore, a multi-blade configuration is expected to be means for achieving lower noises.
However, in cases where the number of the blade parts is more than six, when the inlet angle of the blade parts is made small such that the blade parts are shaped in a reclining manner, the neighboring blade parts of the inducer overlap with each other. Thus, it has been a problem that the formation is impossible using the radial sliding-core as shown in FIGS. 15A and 15B, causing a large restriction on the shape to be formed.
Moreover, in the conventional configuration shown in FIG. 16, even when the number of the blade parts of inducer 31 is increased, the formation is possible because inducer 31 is configured with two vertical parts. However, since nut 32 tightens and secures first inducer 31a and second inducer 31b together, the tightening force by nut 32 is also applied to first inducer 31a. Therefore, unless the thickness of first inducer 31a is made thick to some extent or more, first inducer 31a is possibly broken. This causes first inducer 31a to be difficult to thin.
Moreover, increased thickness of first inducer 31a increases the pressure surfaces of the blade parts of first inducer 31a, which causes the root parts of the blade parts to be subjected to the force caused by air resistance. This requires countermeasures such as ones in which the blade parts are made thicker at around the root parts. As a result, there has been a problem that the cross-section area of a passage in inducer 31 becomes narrow, resulting in a reduced air-blowing efficiency.
Moreover, since the thickness of first inducer 31a is large, the blade parts overlap with each other in the vertical direction when the number of the blade parts is large and the inlet angle of the blade parts is small. For this reason, there has been another problem that the formation of the inducer is impossible using a simple two-plate mold composed of a cavity and a core. In addition, the conventional electric blower has been provided with no countermeasures of preventing the blade parts from moving out of position in the direction of rotary shaft 33 and in the direction along a circumference of rotary shaft 33.
Moreover, in the conventional configuration shown in FIGS. 17A and 17B, first inducer 41a and second inducer 41b are fitted with each other by shrinkage-fit. This allows the smaller thickness of first inducer 41a; however, it becomes impossible to form first inducer 41a and second inducer 41b using a resin. For this reason, there has been a problem that the configuration is not suitable for products manufactured in volume production.
In addition, the fitting of projections 44 with recesses 43 prevents first inducer 41a from moving out of position in the direction along the circumference of the rotary shaft. In the direction of the rotary shaft toward second inducer 41b, it is possible to prevent the first inducer from moving out of position because blade parts 42a hit blade parts 42b. However, when being exposed to force in the opposite direction, first inducer 41a possibly moves out of position in the direction along the circumference of the rotary shaft.
In particular, when inducer 41 having such a configuration is employed in an electric blower such as a cleaner, the opposed side to second inducer 41b, i.e. toward the suction side in the electric blower, is negative in pressure. Therefore, first inducer 41a is pulled toward the suction side, which causes the mating surfaces of first inducer 41a and second inducer 41b to move out of position in the direction of the rotary shaft. This has been a problem.    Patent Literature 1: Japanese Patent Unexamined Publication No. 2000-45993    Patent Literature 2: Japanese Patent Unexamined Publication No. S59-103999    Patent Literature 3: Japanese Patent Unexamined Publication No. H05-149103